High temperature columbium base alloys



N Drawing. Filed May 18, 1962, Ser. No. 204,024 8 Claims. (Cl. 75-114 This invention relates to novel columbium base alloys, and more particularly to new and improved columbium base alloys possessing superior stress-rupture strength at high temperatures.

The principal limitation in gas turbine technology today is the maximum turbine inlet temperature. The turbine inlet temperature is in turn limited by the temperature that the turbine vanes and blades are able to withstand without danger of failure. Formerly, the best available high temperature alloys were nickel andc'obalt base superalloys, but critical structural components, such as turbine vanes and blades constructed from such alloys are limited to maximum operating temperatures of between 1600 and 1900" F.

Among the technically most important physical qualities of columbium as an alloy base are its high melting temperature (4474 F.), and its low neutron-capture cross-section. Columbium' is, therefore, potentially useful for fast aircraft and space flight vehicles and in nuclear reactors.

For many years it has been generally known that the high temperature strength properties of metals are closely related to their melting points. Thus, metals having a high melting point also tend to have high temperature strength potenials.

The need for structural materials for service at temperatures in excess of those obtainable with present materials of construction has stimulated interest in the refractory metals, particularly, chromium, columbium, molybdenum, and tungsten. Until about 1957, molyb denum was considered the chief prospect for such usage. However, at the high temperature service conditions needed, molybdenum oxidizes at a catastrophic rate, principally because molybdenum oxide is volatile at elevated temperatures. Due to the very great problems encountered in attempts to coat molybdenum, and problems in fabricability, interest has recently shifted to columbium as an alloy base for high temperature service.

Columbium is inherently a soft, ductile, readily fabricable material. Although its melting temperature is about 4474 R, pure columbium becomes too weak for structural use at temperatures above 1200 F. Columbium is also a very reactive metal in that it dissolves large quantities of oxygen, and probably nitrogen, on exposure to atmospheres containing even small amounts of these elements at modest temperatures.

Although columbium suffers from oxidation, its oxide does not volatilize, and it is thus potentially possible to localize oxygen attack on columbium by coating the metal. Further advantages offered by columbium base alloys, as compared with molybdenum base alloys, are that columbium alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum.

Until recent years estimated ore reserves of columbium were so small that there was only a mild interest in columbium base alloys. However, with the discovery of substantial ore bodies the potential availability of columbium has become so great that scarcity is no longer a restriction on its use.

I The strength and oxidation resistance of pure colum- Patented July 6, 1965 provide novel and improved columbium base alloys having unusually high stress-nipture strength at temperatures up to at least about 2200 F.

It is another object of this invention to provide new and improved columbium base alloys that achieve superior stress-rupture and tensile strength at temperatures up to at least about 2200 F. without any detrimental effect on either fabricability or room-temperature ductility of the alloys. 1

Another object of this invention is to provide a new and improved columbium base alloy having superior room-temperature tensile strength.

Another object of this invention is to provide a new and improved columbium base alloy having superior high temperature stress-rupture and tensile strength and improved oxidation resistance.

A further object of this invention is to provide a new and improved columbium base alloy that achieves unusually high stress-rupture strength at temperatures up to at least about 2200 F. without the necessity for any heat treatment.

A still further object of this invenion is to provide a new and improved solid solution strengthened columbium base alloy in which the stress-rupture strength and tensile Strength of the solid solution strengthened alloy at temperatures up to at least about 2200 F. is significantly improved by the addition of minor amounts of another element.

Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the compositions and combinations particularly pointed out in the appended claims.

To achieve the foregoing objects, and in accordance with its purpose, this invention provides a composition of matter for achieving a columbium base alloy having superior high temperature stress-rupture and tensile strength. As embodied, this composition comprises a columbium base alloy, such as, for example, the alloys described in copending application Serial Number 65,962, filed October 31, 1960, with a minor addition of the element beryllium. The beryllium content should be less than 0.3 percent, is advantageously between about 0.05 and 0.15 percent, and for best results should be about 0.1 percent.

Also in accordance with this invention, the addition of inor amounts of chromium to the foregoing compositions is beneficial in improving oxidation resistance. For this purpose the chromium content may be from 0.3 to 4 percent, is advantageously from 0.5 to 2 percent, and is preferably about 1 percent.

The alloys of this invention may be advantageously formed by minor additions of beryllium or beryllium plus chromium, in the amounts described above, to columbium base alloys consisting of columbium and from 1 to 40 percent by weight of at least'one element selected from the group consisting of tantalum, tungsten, and molybdenum. Such alloys may also include in combination with the foregoing ingredients up to 10 percent of at least consumable or non-consumable electrodes.

beryllium additions in improving stress-rupture behavior of columbium base alloys are not well understood, it is believed that the improvement in stress-rupture properties achieved with minor beryllium additions is attained through a second-phase strengthening mechanism. The addition of minor amounts of beryllium in accordance with this invention has no effect on oxidation resistance of the alloy base.

In preparation of the novel alloys of this invention and in preparation of the examples which follow, known melting and casting techniques were used.

For a clearer understanding of the invention, specific examples of the invention are given below. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention.

To insure'homogeneity of the alloys, it is desirable to use multiple melting. Individual melts can be melt-cast together, and the melt may be permitted to cool and solidify into a predetermined shape.

In operation, the melting can be achieved by either an induction type furnace or an arc furnace using 'either An arc melt ing furnace using a chilled copper crucible has been used advantageously.

Regardless of the type of furnace used, great care must be taken in the melting and casting operation to protect the molten metal from atmospheric contamination by oxygen, nitrogen, and other gases which may be present, and the melting operation is, accordingly, conducted in a controlled atmosphere.

In preparing the metals for the charge, almost any appropriate form of the metals may be used, such as, shot, granules, powder, or wire. Ideally, the metals should be'of the highest purity commercially obtainable. The alloy obtained will provide a workable metal which can be used in high temperature applications and retain high strength at temperatures well in excess of those to which present day high temperature alloys can be subjected.

The alloys of this invention have good stress-rupture and tensile strength properties at temperatures as high as at least about 2200 F.

Example I An ingot of percent by weight of tungsten, 0.1 percent of beryl1ium,'and the balance essentially of columbium was prepared in a water-cooled copper crucible of a non-consumable electrode-type melting furnace using a tungsten electrode. Prior to melting the furnace chamber was evacuated three times and then backfilled with helium to provide a sufiiciently inert air-free controlled atmosphere. The charge of this example was melted nine times with the total number of melts needed for homogeneity being determined by operator observation and radiography. When the charge had been melted for the last time, the furnace was turned off and the melt was-allowed to cool in the helium atmosphere, discharged from the crucible and tested as described below.

The testing procedures used were as follows:

An are cast 2 /2 x A x i inches button ingot was machined fiat on the ends and sides to 2% x x '95 inches. After machining the arc-cast ingot was encased in thin sheets of molybdenum. The sheathed ingots were inserted in tight fitting holes machined in a boiler plate yoke assembly. Stainless steel cover plates were welded on the top and bottom of the yoke and a low carbon steel evacuation tube was also welded in place. The gas tight yoke cover assembly was then evacuated to approximately microns at 1800 F. and sealed by forging. Final ly, the assembly was rolled at 1800 F. to an overall thickness of /6 inch.

After rolling, the alloy strips now measuring about 8 x x .09 inches, were recovered. The strips were then conditioned by grinding and pickling, andtheir hardness was measured. Qualitative ratings of fabricability were made by visual observation. The fabricability of this alloy at 1800 F. was excellent.

The alloy was then process-annealed for V2 hour at 3000 F., and its hardness was measured. Next a check for final rolling temperature was made using small sam ples. Tests were made at room tmperature, 500 F., and 1000 F. or 1800 F. Finally, the strips were rolled to .03 inch thickness at the lowest permissible temperature and the hardness was again measured. For this particular alloy a good fabrication temperature was 1800 F.

Coupons of the alloy were machined and stress relieved for /2 hour at 2200 F. in an argon atmosphere. These coupons were then tested to failure in tension at both room temperature and at 2200 F.

The room temperature tests were performed on a Southwark universal testing machine at cross-head speeds of 0.005 inch per minute to the yield load, and at a rate of 0.05 inch per minute to failure.

The 2200 F. tests were performed in a vacuum of 1 micron in a creep-test rack. The specimens were wrapped with tantalum foil for gettering purposes. The approximate time of exposure to temperatures in excess of 1800 F. was /2 hour. In these tests, the ultimate tensile strength of this alloy at room temperature was 120,000 p.s.i. and its ultimate tensile strength at 2200 F. was 48,000 p.s.i. Ductility of the alloy at room temperature was 24 percent elongation.

Stress-rupture tests were performed on specimens of the alloy at 2200 F. in a vacuum of about 0.1 micron. For these tests the specimens were again wrapped in tantalum foil. The stress-rupture strength of this alloy at 2200 F. was 19,000 p.s.i. for 7.4 hours.

For oxidation tests a specimen was exposed to a staticair-atmosphere at 2200 F. for /2 hour. Specimens were prepared by grinding to a uniform finish with a 240-grit abrasive. After carefully weighing and measuring all critical components, the specimens were exposed in prefired porcelain crucibles in such a manner that the maximum surface exposure was achieved. After exposure each specimen was evaluated by measuring weight gain (metal plus oxide), weight loss (metal plus adherent oxide), metallographic determination of metal loss, and microhardness traverse to determine the depth of contamination.

In these tests, contamination was assumed when hardness was more than 50 VHN greater than base hardness. For this alloy, the weight gain (oxide plus metal) in mg./cm. was 20 and the weight loss (brushed to remove non-adherent oxide) was 37. The depth of contamination was 43 mils and extended through the center of the specimen.

The as cast hardness of this alloy was 257 VHN. The hardness after breakdown or initial fabrication was 252 VHN, and the hardness after process annealing at 3000 F. for /2 hour was 255 VHN.

The tensile and stress-rupture strength properties of this alloy, incorporating 0.1 percent beryllium, were distinctly superior to those of a control alloy consisting of the same base, i.e., 15. percent tungsten by weight and balance essentially columbium. The ultimate tensile strength of the control Cb-l5 W alloy at room temperature was 97,000 p.s.i. and at 2200 F. was 31,000 p.s.i., whereas in the Cb-lS W-0.1 Be alloy of this invention, the ultimate tensile strength at room temperature was 120,000 p.s.i. and at 2200 F. was 48,000 p.s.i.

The stress-rupture strength of the Cb-lS W base at 2200" F. was about 1 hour at 18,000 p.s.i., whereas the stress-rupture strength of the Cb-lS W-0.l Be alloy of this invention at 2200 F. was 7.4 hours at 19,000 p.s.i. The room temperature ductility of the Cb-l5 W base was 22 percent elongation, whereas the room temperature ductility of the Cb-l5 W-0.l Be alloy was 24 percent elongation. Thus, in spite of the markedly improved strength characteristics of the alloy attributable to the beryllium addition of this invention, this addition causes no decline in the room-temperature ductility of the alloy.

Example II An ingot of a columbiurn, tungsten, chromium, beryllium metal alloy composition consisting by weight of percent tungsten, 1 percent chromium, 0.1 percent beryllium, and the balance essentially columbium was prepared as described in Example I.

The tests of this alloy were performed in the same manner as the tests set forth in Example I. This alloy was melted eleven times.

Although this alloy was not tested for ultimate tensile strength at either room temperature or 2200 F., the stressrupture strength at 2200 F. was 9.5 hours at 19,000 p.s.i.

The initial fabricability of this alloy was rated as fair after fabrication at 1800 F., but its final fabricability was rated as very good after final fabrication at 1800 F. with a process anneal of /2 hour at 3000 F. administered between initial and final fabrication.

The as cast hardness of this alloy was 332 VHN; its hardness after initial fabrication or breakdown was 278 VHN; and its hardness after process annealing was 237 VHN.

The results of oxidation testing on this alloy were as follows: Weight gain in mg./cm. (oxide plus metal) was 15; weight loss (brushed to remove nonadherent oxide) was 23; the depth of contamination was 31 mils; and the center of the specimen was not contaminated. There was thus a distinct improvement in oxidation resistance of Example 11 over that of Example I.

In accordance with this invention, columbium base alloys are thus provided that achieve distinctly superior strength qualities, both in tensile and stress-rupture strength, over the base alloys by minor additions of beryllium or beryllium and chromium.

The invention in its broader aspects is not limited to I the specific details shown and described, but departures may be made from such details within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A columbium base alloy consisting essentially of,

by weight: from 1 to 40 percent of at least one element selected from the group consisting of tantalum, tungsten, and molybdenum; up to 10 percent of at least one element selected from the group consisting of hafnium, zirconium, and vanadium; up to 4 percent chromium; between about 0.05 and 0.03 percent beryllium; and balance essentially columbium.

2. A columbium base alloy consisting essentially of, by

weight: from 1 to 40 percent of at least one element selected from the group consisting of tantalum, tungsten, and molybdenum; up to 10 percent of at least one element selected from the group consisting of hafnium, zirconium, and vanadium; up to 4 percent chromium; between about 0.05 and 0.15 percent beryllium; and balance essentially columbium.

3. A columbium base alloy consisting essentially of, by weight: from 1 to 40 percent of at least one element selected from the group consisting of tantalum, tungsten, and molybdenum; up to 10 percent of at least one element selected from the group consisting of hafnium, zirconium, and vanadium; up to 4 percent chromium; about 0.1 percent beryllium, and balance essentially columbium.

4. A columbium base alloy consisting essentially of, by weight: 5 to 35 percent tungsten, between about 0.05 and 0.3 percent beryllium, and balance essentially columbium.

5. A columbium base alloy consisting essentially of,

by weight: 5 to 35 percent tungsten, between about 0.05 and 0.15 percent beryllium, and balance essentially columbium.

6. A columbium base alloy consisting essentially of, by weight: 5 to 35 percent tungsten, about 0.1 percent beryllium, and balance essentially columbium.

7. A columbium base alloy consisting essentially of, by weight: .15 percent tungsten, 0.1 percent beryllium, and balance essentially columbium.

8. A columbiumbase alloy consisting essentially of, by weight: 15 percent tungsten, 1 percent chromium, 0.1 percent beryllium, and balance essentially columbium.

References Cited by the Examiner UNITED STATES PATENTS 4/59 Wainer -174 4/62 Wlodek et al. 75-174- DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, WINSTON A. DOUGLAS,

Examiners.

UNITED STATES PATENT OFFICE q CERTIFICATE OF CORRECTION Patent No. 3,193,385

Robert I. Jaffee et a1.

July 6, 1965 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 6, for "0.03" read 0.3

Signed and sealed this 27th day of September 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

1. A COLUMBIUM BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT: FROM 1 TO 40 PERCENT OF AT LEAST ONE ELEMENT SELECTED FROMTHE GROUP CONSISTING OF TANTALUM, TUNGSTEN, AND MOLYBDENUM; UP TO 10 PERCENT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF HAFINIUM, ZIRCONIUM, AND VANADIUM; UP TO 4 PERCENT CHROMIUM; BETWEEN ABOUT 0.05 AND 0.03 PERCENT BERYLLIUM; AND BALANCE ESSENTIALLY COLUMBIUM.
 4. A COLUMBIUM BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT: 5 TO 35 PERCENT TUNGSTEN, BETWEEN ABOUT 0.05 AND 0.3 PERCENT BERYLLIUM, AND BALANCE ESSENTIALLY COLUMBIUM. 